The invention relates to a circuit which is operated alternately with at least two frequencies carried by a common connection while using filters.
For example, intermediate frequency amplifiers with amplitude-modulated and frequency-modulated signals are used in radio receivers. Only a single amplifier should be used for AM and FM owing to the cost. Thus when using electrical filters with transformers, the coupling windings of the two transformers are connected in series and this series connection is connected to the input connection of the IF amplifier. When operating the IF amplifier at one or the other frequency, the secondary winding of the filter, which is not in operation and is connected in series with the secondary winding of the active transformer, is so low in resistance that the series resistance caused thereby in each case is negligibly small.
When using ceramics filters the principle of connection mentioned cannot easily be used because the transmission characteristics of the ceramic filters are only guaranteed when terminating them with the desired terminating resistance. Ceramics filters were therefore only used, for a long time, when the respective filter required was connected separately to the input or to the output of an amplifier during frequency change with the aid of an additional switch.